Well, in at least some cases, they do understand what they're doing, on a deep intuitive level!

Using spaced repetition to see through a piece of mathematics

https://news.ycombinator.com/item?id=18895613

The title is a slight misnomer in my opinion. The author did make heavy use of spaced repetition of course, but deemed it neither necessary nor sufficient for the result he described.

The actual active ingredient is curiosity to the degree of obsession. Actually wanting to understand what's going on.

You can do that with pure math just as much as with applied.

But it may be harder for many people for such a desire to arise if there is no obvious connection to real-world matters. (I certainly had such thoughts, sometimes! What is this for? What's the point?)